As explained, the severity is the impact on the system. For example, a null pointer that causes the system to crash will have a critical severity.
But maybe this bug occurs very rarely and in very specific circumstances.
On the other hand you have also found another bug that makes your app use bright green Comic Sans. It is not severe as it is only cosmetic.
But for some reason, on your last release this happens every time the app is started.
It seems likely that the second issue will have a much higher priority and will have to be fixed immediately, while the null pointer can wait.
Severity is purely technical. Priority is a project/commercial decision (which is informed by severity but also other considerations). They generally do not align.
But maybe this bug occurs very rarely and in very specific circumstances.
On the other hand you have also found another bug that makes your app use bright green Comic Sans. It is not severe as it is only cosmetic.
But for some reason, on your last release this happens every time the app is started.
It seems likely that the second issue will have a much higher priority and will have to be fixed immediately, while the null pointer can wait.
Severity is purely technical. Priority is a project/commercial decision (which is informed by severity but also other considerations). They generally do not align.